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Process Engineering
RHEOLOGICAL PROPERTIES OF WATER-IN-BRAZILIAN CRUDE OIL EMULSIONS: EFFECT OF WATER CONTENT, SALINITY AND pH. Cesar B.Z de Oliveira, Walisson Souza, Camlia F Santana, Cesar Costapinto Santana, Cláudio Dariva, Elton Franceschi, Ricardo Andre Guarnieri, Montserrat Fortuny, and Alexandre Ferreira Santos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01227 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018
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Energy & Fuels
RHEOLOGICAL PROPERTIES OF WATER-IN-BRAZILIAN CRUDE OIL EMULSIONS: EFFECT OF WATER CONTENT, SALINITY AND pH.
Cesar B. Z. de Oliveira1, W.J. Souza2, C. F. Santana3 ,C. C. Santana3, C. Dariva3, E. Franceschi3,R. A. Guarnieri4, M. Fortuny5and A. F. Santos5
1
Centro Universitário Christus, Brazil 2
3
4
Faculdade Pio Décimo, Brazil
Núcleo de Estudos em Sistemas Coloidais, ITP, PEP, Universidade Tiradentes (UNIT)
Centro de Pesquisas e Desenvolvimento Leopoldo A.Miguez de Mello (Cenpes), Petrobras 5
Departamento de Engenharia Química (DEQ), Universidade Federal do Paraná (UFPR)
To whom all correspondence should be addressed. Walisson de Jesus Souza Faculdade Pio Décimo/ AvenidaPresidente Tancredo Neves 5655, Aracaju, 49075-010 Sergipe (SE),Brazil. Phone ; 079 98849-8625 Email:
[email protected] ACS Paragon Plus Environment
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Abstract During the crude oil production, stable water-in-oil (W/O) emulsions are generally found along the pipeline between the reservoir and surface facilities. The main objective of this work is discussing about the influence of the salinity (with NaCl at 0, 50 and 100 g.L-1), pH (2, 6 and 10 values) and water content (variations between 8% and 65%) upon the rheological behavior of water-in-crude oil emulsions. Tests including the rotational and oscillatory rheology for assessment of profiles of viscosity, elastic and viscous modulus for the different cases were performed. Semi-theoretical models and available empirical literature models have been successfully used to fit the rheological data, enabling a better understanding of the interactions between the constituents of the samples at a colloidal level. Results presented here show that the emulsion loss modulus calculated from bulk rheological tests do not respond differently for acid, quasi-isoelectric point and alkaline aqueous phase; this modulus also has a salt contentindependent behavior for crude oils with different content of resins and asphaltenes. The presence of electrolyte in the emulsions may decrease the storage modulus, once the concentration of resins and in special asphaltenes were low. For higher content of these molecules, the attractive electrostatic forces are not enough to change the interfacial tension between the macromolecules.
Keywords: water-in-crude oil emulsion; viscosity; viscoelasticity; water content.
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1. Introduction
Depending on the characteristics of the reservoir and the methods employed in the crude oil recovery, the crude oil production is accompanied by different amounts of water which may promote the formation of stable water-in-crude oil emulsions (W/O) along the pipeline from reservoir to the surface facilities. Several studies are found in literature focused on the behavior of emulsions, and in particular, to understanding how the stability of those emulsions may vary with the chemical and physical properties.1 It is well known that the stability and viscosity of the emulsions are directly related to the chemical composition and rheological properties of the crude oil, as well by volume fraction of the disperse phase2.Furthermore, emulsion stability is usually described by classical mechanisms and phenomena in literature, including van der Waals attractive forces3,steric repulsion4, the Gibbs-Marangoni effect5, and interfacial rigid film formation2. From the chemical point of view, the composition of the crude oils is commonly classified by the SARA analysis, which separates the families of compounds in saturates, aromatics, resins and asphaltenes. Among these families, it is well accepted that resins and asphaltenes are the most surface active species and ultimately the main responsible for the stabilization of water-in-crude oil emulsions6.Besides, the composition and structure of the disperse phase – droplet size distribution (DSD)7-8,salinity9, pH10, may also play a role over the emulsion viscosity and over its stability, making the emulsion processing cumbersome. Past over 20 years, several reports have been published regarding the relative viscosity of emulsions for a number of systems, including diluted and slightly concentrated crude oil
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emulsions (with less than 60% v/v), and concentrated emulsions11-15. These papers usually describe the rheological properties of water-in-crude oil emulsions as a function of the volume fraction of the disperse phase, where the emulsions viscosity is normally used as the relative
viscosity ( ), which represents the ratio between the viscosity of the emulsion ( ) to the viscosity of the continuous phase ( ). Several works had found good correlation through the
semi-empirical exponentials models as the proposed by Mooney, Krieger & Dougherty, and by Pal & Rhodes.11-14 The viscoelastic properties of emulsions were investigate as an important rheological parameter to predict the stability of emulsion and their mechanical behavior over shearflow.7,8,12,16-18 Being possible to know the effects of shearing on the deformation of the water droplets and their coalescence. Aomari et al.12employed a stress controlled rheometer and evaluated the viscosity and the viscoelastic behavior of water in light Arabian crude oil in presence and free of an additional surfactant (SPAN 80). They observed the increase of viscosity with the increase of the water content, following an exponential relationship. The dynamic shear stress sweep presented increase of the loss modulus in function of the shear stress at constant frequency (f=1Hz) with the increase of the water content. The emulsions were in the viscoelastic regime only for lower shear stresses. They observed the predominance of the loss modulus for emulsions with lower water content, and for higher water content the principal shear modulus was the storage modulus, which determines the increase of the emulsion elasticity. They credited the behavior with the increase of steric interactions on the layers at the water/oil interface and due to the proximity of the droplets inside the continuous phase. Quintero et al.14realized full rheological characterization in respect to the water content of water-in-oil emulsions, i.e. flow and dynamic, from diluted to high concentrated emulsions which mean equal/less 60% v/v and
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70% v/v, respectively. They found results of viscosity and mechanical properties very similar to Aomari et al.12,with the increase of the storage modulus as a function of the water content, up to the maximum volume fraction. Some groups have pointed out that the rheological properties studies of water-in-oil emulsions allow one to understand the flowing conditions of emulsions, and their stability. In many cases, emulsion stability and rheological behavior are interconnected, because the coalescence depends on the rheological properties of the interface as well as the size and concentration of solids and molecules in the interface or near the interface. To deal with emulsion formation problems during crude oil production, it is a common practice in the petroleum field to realize lab tests to analyze the emulsion rheological behavior and stability, which is not a satisfactory approach. The main objective of this work is to evaluate the effect of water content, salinity and pH on the bulk viscoelastic properties of the emulsions. The experiments were focused on water-incrude oil emulsions of two Brazilian crude oils. The novelty of this article is the use of theoretical approaches to understand the relationship of the bulk viscoelastic behavior of crude oil emulsions with a wide set of emulsion properties. Besides, a large rheological databank for two crude oils is discussed on the basis of the chemical composition, providing insight into the flow assurance connected issues.
2. Theory Section 2.1. Emulsion viscosity predict models
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There are several researches in literature using semi-empiric models to predict the relative viscosity of emulsions ( ), where the relative viscosity is defined by the ratio between the
viscosity of the emulsion ( ) and the viscosity of the continuous phase ( ). One of the principal factors studied to predict the emulsion viscosity is the volume fraction of the disperse phase ( ). Einstein presented a thermodynamic model for highly diluted colloidal systems ( ≤ % /
)and proposed that the relative viscosity has a linear relationship with the volume fraction; this
model was based on the hydrodynamic interactions and adopted that the droplets, where is the shape factor and is equal to 2.5 for rigid spheres dispersions
η = 1 + αϕ
(1)
When the volume fraction of the disperse phase increases, the hydrodynamics interactions between the droplets became more important and the Einstein’s model loose validity.19On the other hand, a functional modification of the Einstein’s model given by an exponential model allows predicting the relative viscosities of slightly concentrated and concentrated emulsion.
=
$ 1 − "#
(2)
where, is the shape fator (present in the Einstein model) and %& is defined by &/ ' ( ' is
the maximum packing fraction).19
The parameter maximum packing fraction is related to the infinity viscosity of an emulsion. For solid and monodisperse dispersions, this parameter can assume a maximum value of 0.74 which is related with face-centered cubic packing, this structure results in the immobilization of the system due the high density of spheres present in the emulsion.19 For real
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emulsions values close to 0.64-0.68are usually attributed. Quintero et al.14 found ' values of 0.63 and 0.68 for concentrated water-in-light crude oil emulsions at 30ºC.
A few years later, Krieger and Dougherty (1959, Eq. 3) developed an empirical model for concentrated emulsions, considering the maximum packing fraction.
= 1 −
(
)*+,
$
(3)
where, is the shape factor and ϕ- is the maximum packing fraction.20
In 1989, Pal & Rhodes developed an empirical model to predict the viscosity of water-in-
crude oil emulsions with Newtonian and non-Newtonian behavior. The relative viscosity was obtained empirically and validated by theoretical principles, with . / is an adjustment factor of
the model and stands for equal to the concentration of the disperse phasewere the relative viscosity is equal to 100 - "0 = ( = 100)20:
= 21 +
#
34
1,187 −
#
34
0.:;0
8
(4)
2.2. Viscoelastic Properties of Emulsions
The principal mechanical properties are the storage modulus (
